Methods of enhancing separation of primary reflection signals and noise in seismic data using radon transformations

ABSTRACT

Improved methods of processing seismic data which comprise amplitude data assembled in the offset-time domain in which primary reflection signals and noise overlap are provided for. The methods include the step of enhancing the separation between primary reflection signals and coherent noise by transforming the assembled data from the offset-time domain to the time-slowness domain. More specifically, the assembled amplitude data are transformed from the offset-time domain to the time-slowness domain using a Radon transformation according to an index j of the slowness set and a sampling variable Δp; wherein 
               j   =         p   max     -     p   min     +     1   ⁢           ⁢   μ   ⁢           ⁢   sec   ⁢     /     ⁢   m         Δ   ⁢           ⁢   p         ,         
Δp is from about 0.5 to about 4.0 μsec/m, p max  is a predetermined maximum slowness, and p min  is a predetermined minimum slowness. Alternately, an offset weighting factor x n  is applied to the assembled amplitude data, wherein 0&lt;n&lt;1, and the amplitude data are transformed with a Radon transformation. The assembled amplitude data also may be transformed with a Radon transformation applied within defined slowness limits p min  and p max , where p min  is a predetermined minimum slowness and p max  is a predetermined maximum slowness, with a hyperbolic Radon transformation, or, as applied to data which have been uncorrected for normal moveout, with a hyperbolic or parabolic Radon transformation. Thus, the primary reflection signals and coherent noise transform into different regions of the time-slowness domain to enhance the separation therebetween.

CLAIM TO PRIORITY

This application is a continuation of an application of John M.Robinson, Rule 47(b) applicant, entitled “Removal of Noise from SeismicData Using High Resolution Radon Transformations,” U.S. Ser. No.10/238,366, filed Sep. 10, 2002, now U.S. Pat. No. 6,987,706 which is acontinuation-in-part of an application of John M. Robinson, Rule 47(b)Applicant, entitled “Removal of Noise from Seismic Data Using ImprovedRadon Transformations,” U.S. Ser. No. 10/232,118, filed Aug. 30, 2002,now U.S. Pat. No. 6,691,039.

BACKGROUND OF THE INVENTION

The present invention relates to processing of seismic datarepresentative of subsurface features in the earth and, moreparticularly, to improved methods of processing seismic data using highresolution Radon transformations to remove unwanted noise frommeaningful reflection signals.

Seismic surveys are one of the most important techniques for discoveringthe presence of oil and gas deposits. If the data are properly processedand interpreted, a seismic survey can give geologists a picture ofsubsurface geological features, so that they may better identify thosefeatures capable of holding oil and gas. Drilling is extremelyexpensive, and ever more so as easily tapped reservoirs are exhaustedand new reservoirs are harder to reach. Having an accurate picture of anarea's subsurface features can increase the odds of hitting aneconomically recoverable reserve and decrease the odds of wasting moneyand effort on a nonproductive well.

The principle behind seismology is deceptively simple. As seismic wavestravel through the earth, portions of that energy are reflected back tothe surface as the energy waves traverse different geological layers.Those seismic echoes or reflections give valuable information about thedepth and arrangement of the formations, some of which hopefully containoil or gas deposits.

A seismic survey is conducted by deploying an array of energy sourcesand an array of sensors or receivers in an area of interest. Typically,dynamite charges are used as sources for land surveys, and air guns areused for marine surveys. The sources are discharged in a predeterminedsequence, sending seismic energy waves into the earth. The reflectionsfrom those energy waves or “signals” then are detected by the array ofsensors. Each sensor records the amplitude of incoming signals over timeat that particular location. Since the physical location of the sourcesand receivers is known, the time it takes for a reflection signal totravel from a source to a sensor is directly related to the depth of theformation that caused the reflection. Thus, the amplitude data from thearray of sensors can be analyzed to determine the size and location ofpotential deposits.

This analysis typically starts by organizing the data from the array ofsensors into common geometry gathers. That is, data from a number ofsensors that share a common geometry are analyzed together. A gatherwill provide information about a particular spot or profile in the areabeing surveyed. Ultimately, the data will be organized into manydifferent gathers and processed before the analysis is completed and theentire survey area mapped.

The types of gathers typically used include: common midpoint, where thesensors and their respective sources share a common midpoint; commonsource, where all the sensors share a common source; common offset,where all the sensors and their respective sources have the sameseparation or “offset”; and common receiver, where a number of sourcesshare a common receiver. Common midpoint gathers are the most commongather today because they allow the measurement of a single point on areflective subsurface feature from multiple source-receiver pairs, thusincreasing the accuracy of the depth calculated for that feature.

The data in a gather are typically recorded or first assembled in theoffset-time domain. That is, the amplitude data recorded at each of thereceivers in the gather are assembled or displayed together as afunction of offset, i.e., the distance of the receiver from a referencepoint, and as a function of time. The time required for a given signalto reach and be detected by successive receivers is a function of itsvelocity and the distance traveled. Those functions are referred to askinematic travel time trajectories. Thus, at least in theory, when thegathered data are displayed in the offset-time domain, or “X-T” domain,the amplitude peaks corresponding to reflection signals detected at thegathered sensors should align into patterns that mirror the kinematictravel time trajectories. It is from those trajectories that oneultimately may determine an estimate of the depths at which formationsexist.

A number of factors, however, make the practice of seismology and,especially, the interpretation of seismic data much more complicatedthan its basic principles. First, the reflected signals that indicatethe presence of geological strata typically are mixed with a variety ofnoise.

The most meaningful signals are the so-called primary reflectionsignals, those signals that travel down to the reflective surface andthen back up to a receiver. When a source is discharged, however, aportion of the signal travels directly to receivers without reflectingoff of any subsurface features. In addition, a signal may bounce off ofa subsurface feature, bounce off the surface, and then bounce off thesame or another subsurface feature, one or more times, creatingso-called multiple reflection signals. Other portions of the signal turninto noise as part of ground roll, refractions, and unresolvablescattered events. Some noise, both random and coherent, is generated bynatural and man-made events outside the control of the survey.

All of this noise is occurring simultaneously with the reflectionsignals that indicate subsurface features. Thus, the noise andreflection signals tend to overlap when the survey data are displayed inX-T space. The overlap can mask primary reflection signals and make itdifficult or impossible to identify patterns in the display upon whichinferences about subsurface geology may be drawn. Accordingly, variousmathematical methods have been developed to process seismic data in sucha way that noise is separated from primary reflection signals.

Many such methods seek to achieve a separation of signal and noise bytransforming the data from the X-T domain to other domains. In otherdomains, such as the frequency-wavenumber (F-K) domain or thetime-slowness (tau-P), there is less overlap between the signal andnoise data. Once the data are transformed, various mathematical filtersare applied to the transformed data to eliminate as much of the noise aspossible and, thereby, to enhance the primary reflection signals. Thedata then are inverse transformed back into the offset-time domain forinterpretation or further processing.

For example, so-called Radon filters are commonly used to attenuate orremove multiple reflection signals. Such methods rely on Radontransformation equations to transform data from the offset-time (X-T)domain to the time-slowness (tau-P) where it can be filtered. Morespecifically, the X-T data are transformed along kinematic travel timetrajectories having constant velocities and slownesses, where slowness pis defined as reciprocal velocity (or p=1/v).

Such prior art Radon methods, however, typically first process the datato compensate for the increase in travel time as sensors are furtherremoved from the source. This step is referred to as normal moveout or“NMO” correction. It is designed to eliminate the differences in timethat exist between the primary reflection signals recorded at close-inreceivers, i.e., at near offsets, and those recorded at remotereceivers, i.e., at far offsets. Primary signals, after NMO correction,generally will transform into the tau-P domain at or near zero slowness.Thus, a mute filter may be defined and applied in the tau-P domain. Thefilter mutes, i.e., excludes all data, including the transformed primarysignals, below a defined slowness value p_(mute).

The data that remain after applying the mute filter contains asubstantial portion of the signals corresponding to multiplereflections. That unmuted data are then transformed back intooffset-time space and are subtracted from the original data in thegather. The subtraction process removes the multiple reflection signalsfrom the data gather, leaving the primary reflection signals morereadily apparent and easier to interpret.

It will be appreciated, however, that in such prior art Radon filters,noise and multiple reflection signals recorded by receivers close to thegather reference point (“near trace multiples”) are not as effectivelyseparated from the primary reflections. The lack of separation in largepart is an artifact of the NMO correction performed prior totransformation. Because NMO correction tends to eliminate offset timedifferences in primary signals, primary signals and near trace multiplesignals both transform at or near zero slowness in the tau-P domain.When the mute filter is applied, therefore, the near trace multiples aremuted along with the primary signal. Since they are muted, near tracemultiples are not subtracted from and remain in the original data gatheras noise that can mask primary reflection signals.

Radon filters have been developed for use in connection with commonsource, common receiver, common offset, and common midpoint gathers.They include those based on linear slant-stack, parabolic, andhyperbolic kinematic travel time trajectories. The general case forwardtransformation equation used in Radon filtration processes,R(p,τ)[d(x,t)], is set forth below:

u(p, τ) = ∫_(−∞)^(∞)𝕕x∫_(−∞)^(∞)𝕕t d(x, t)δ[f(t, x, τ, p)](forward  transformation)where

-   -   u(p,τ)=transform coefficient at slowness p and zero-offset time        τ    -   d(x,t)=measured seismogram at offset x and two-way time t    -   p=slowness    -   t=two-way travel time    -   τ=two-way intercept time at p=0    -   x=offset    -   δ=Dirac delta function    -   ƒ(t,x,τ,p)=forward transform function

The forward transform function for linear slant stack kinematic traveltime trajectories is as follows:

f(t, x, τ, p) = t − τ − p x where $\begin{matrix}{{\delta\left\lbrack {f\left( {t,x,\tau,p} \right)} \right\rbrack} = {\delta\left( {t - \tau - {p\; x}} \right)}} \\{{= 1},{{{when}\mspace{14mu} t} = {\tau + {p\; x}}},{and}} \\{{= 0},{{elsewhere}.}}\end{matrix}$Thus, the forward linear slant stack Radon transformation equationbecomes

u(p, τ) = ∫_(−∞)^(∞)𝕕x d(x, τ + p x)

The forward transform function for parabolic kinematic trajectories isas follows:

f(t, x, τ, p) = t − τ − p x² where $\begin{matrix}{{\delta\left\lbrack {f\left( {t,x,\tau,p} \right)} \right\rbrack} = {\delta\left( {t - \tau - {p\; x^{2}}} \right)}} \\{{= 1},{{{when}\mspace{14mu} t} = {\tau + {p\; x^{2}}}},{and}} \\{{= 0},{{elsewhere}.}}\end{matrix}$Thus, the forward parabolic Radon transformation equation becomes

u(p, τ) = ∫_(−∞)^(∞)𝕕x d(x, τ + p x²)

The forward transform function for hyperbolic kinematic travel timetrajectories is as follows:

${f\left( {t,x,\tau,p} \right)} = {t - \sqrt{\tau^{2} - {p^{2}x^{2}}}}$where $\begin{matrix}{{\delta\left\lbrack {f\left( {t,x,\tau,p} \right)} \right\rbrack} = {\delta\left( {t - \sqrt{\tau^{2} + {p^{2}x^{2}}}} \right)}} \\{{= 1},{{{when}\mspace{14mu} t} = \sqrt{\tau^{2} + {p^{2}x^{2}}}},{and}} \\{{= 0},{{elsewhere}.}}\end{matrix}$Thus, the forward hyperbolic Radon transformation equation becomes

${u\left( {p,\tau} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}x}\;{d\left( {x,\sqrt{\tau^{2} + {p^{2}x^{2}}}} \right)}}}$

A general case inverse Radon transformation equation is set forth below:

d(x, t) = ∫_(−∞)^(∞)𝕕p∫_(−∞)^(∞)𝕕τρ(τ) * u(p, τ)δ[g(t, x, τ, p)](inverse  transformation)where

-   -   g(t,x,τ,p)=inverse transform function, and    -   ρ(τ)*=convolution of rho filter.

The inverse transform function for linear slant stack kinematictrajectories is as follows:g(t,x,τ,p)=τ−t+pxThus, when τ=t−px, the inverse linear slant stack Radon transformationequation becomes

d(x, t) = ∫_(−∞)^(∞)𝕕p ρ(τ) * u(p, t − p x)

The inverse transform function for parabolic trajectories is as follows:g(t,x,τ,p)=τ−t+px ²Thus, when τ=t−px², the inverse parabolic Radon transformation equationbecomes

d(x, t) = ∫_(−∞)^(∞)𝕕p ρ(τ) * u(p, t − p x²)

The inverse transform function for hyperbolic trajectories is asfollows:g(t,x,τ,p)=τ−√{square root over (t ² −p ² x ²)}Thus, when τ=√{square root over (t²−p²x²)}, the inverse hyperbolic Radontransformation equation becomes

${d\left( {x,t} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}p}\;{\rho(\tau)}*{u\left( {p,\sqrt{t^{2} - {p^{2}x^{2}}}} \right)}}}$

The choice of which form of Radon transformation preferably is guided bythe travel time trajectory at which signals of interest in the data arerecorded. Common midpoint gathers, because they offer greater accuracyby measuring a single point from multiple source-receiver pairs, arepreferred over other types of gathers. Primary reflection signals in acommon midpoint gather generally will have hyperbolic kinematictrajectories. Thus, it would be preferable to use hyperbolic Radontransforms.

To date, however, Radon transformations based on linear slant stack andparabolic trajectories have been more commonly used. The transformfunction for hyperbolic trajectories contains a square root whereasthose for linear slant stack and parabolic transform functions do not.Thus, the computational intensity of hyperbolic Radon transformationshas in large part discouraged their use in seismic processing.

It has not been practical to accommodate the added complexity ofhyperbolic Radon transformations because the computational requirementsof conventional processes are already substantial. NMO correctioninvolves a least-mean-squares (“LMS”) energy minimization calculation.Forward and inverse Radon transforms also are not exact inverses of eachother. Accordingly, an additional LMS calculation is often used in thetransformation. Those calculations in general and, especially LMSanalyses, are complex and require substantial computing time, even onadvanced computer systems.

Moreover, a typical seismic survey may involve hundreds of sources andreceivers, thus generating tremendous quantities of raw data. The datamay be subjected to thousands of different data gathers. Each gather issubjected not only to filtering processes as described above, but in alllikelihood to many other enhancement processes as well. For example,data are typically processed to compensate for the diminution inamplitude as a signal travels through the earth (“amplitude balancing”).Then, once the individual gathers have been filtered, they must be“stacked”, or compiled with other gathers, and subjected to furtherprocessing in order to make inferences about subsurface formations.Seismic processing by prior art Radon methods, therefore, requiressubstantial and costly computational resources, and there is acontinuing need for more efficient and economical processing methods.

It is understood, of course, that the transformation, NMO correction,and various other mathematical processes used in seismic processing donot necessarily operate on all possible data points in the gather.Instead, and more typically, those processes may simply sample the data.For example, the transform functions in Radon transformations are basedon variables t, x, τ, and p. The transformations, however, will notnecessarily be performed at all possible values for each variable.Instead, the data will be sampled a specified number of times, where thenumber of samples for a particular variable may be referred to as theindex for the variable set. For example, k, l, m, and j may be definedas the number of samples in, or the index of, respectively, the time(t), offset (x), tau (τ), and slowness (p) sets. The samples will betaken at specified intervals, Δt, Δx, Δτ, and Δp. Those intervals arereferred to as sampling variables.

The magnitude of the sampling variables, the variable domain and allother factors being equal, will determine the number of operations orsamples that will be performed in the transformation and the accuracy ofthe transformation. As the sampling variables become finer, i.e.,smaller, the number of samples increases and so does the accuracy. Bymaking the sampling variables larger, or coarser, the number of samplesis reduced, but the accuracy of the transformation is reduced as well.

In large part because of the computational intensity of those processes,however, the sampling variables used in prior art Radon processes can berelatively coarse. In NMO correction, for example, industry typicallysamples at Δt values several time greater than that at which the fielddata were recorded, i.e., in the range of 20-40 milliseconds, andsamples at v values of from about 15 to about 120 m/sec. To the extentthat the sampling variables are greater than the data acquisition samplerates, data are effectively discarded and the accuracy of the process isdiminished. In the Radon transformation itself, prior art methodstypically set Δt, Δτ, and Δx at the corresponding sample rates at whichthe data were recorded, i.e., in the range of 2 to 4 milliseconds and 25to 400 meters or more, respectively. Δp is relatively coarse, however,typically being set at values in the range of 4-24 μsec/m. Also, theindex j of the slowness (p) set usually is equal to the fold of theoffset data, i.e., the number of source-receiver pairs in the survey,which typically ranges from about 20 to about 120.

Accordingly, prior art methods may rely on additional computations in anattempt to compensate for the loss of accuracy and resolution inherentin coarse sampling variables. For example, the LMS analysis applied inNMO correction and prior art Radon transformations are believed toobviate the need for finer sampling variables. Certain prior artprocesses utilize trace interpolation, thereby nominally reducing xvalues, but the additional “data” are approximations of what would berecorded at particular offsets between the offsets where receivers wereactually located. Relative inaccuracy in processing individual datagathers also is believed to be acceptable because the data from a largenumber of gathers ultimately are combined for analysis, or “stacked”.Nevertheless, there also is a continuing need for more accurateprocessing of seismic data and processes having finer resolutions.

The velocity at which primary reflection signals are traveling, what isreferred to as the stacking velocity, is used in various analyticalmethods that are applied to seismic data. For example, it is used indetermining the depth and lithology or sediment type of geologicalformations in the survey area. It also is used various seismic attributeanalyses. A stacking velocity function may be determined by what isreferred to as a semblance analysis. Which such methods have providedreasonable estimations of the stacking velocity function for a data set,given the many applications based thereon, it is desirable to define thestacking velocity function as precisely as possible.

An object of this invention, therefore, is to provide a method forprocessing seismic data that more effectively removes unwanted noisefrom meaningful reflection signals.

It also is an object to provide such methods based on hyperbolic Radontransformations.

Another object of this invention is to provide methods for removal ofnoise from seismic data that are comparatively simple and requirerelatively less computing time and resources.

Yet another object is to provide methods for more accurately definingthe stacking velocity function for a data set.

It is a further object of this invention to provide such methods whereinall of the above-mentioned advantages are realized.

Those and other objects and advantages of the invention will be apparentto those skilled in the art upon reading the following detaileddescription and upon reference to the drawings.

SUMMARY OF THE INVENTION

The subject invention provides for methods of processing seismic data toremove unwanted noise from meaningful reflection signals indicative ofsubsurface formations. The methods comprise the steps of obtaining fieldrecords of seismic data detected at a number of seismic receivers in anarea of interest. The seismic data comprise signal amplitude datarecorded over time by each of the receivers and contains both primaryreflection signals and unwanted noise events. The seismic data areassembled into common geometry gathers in an offset-time domain.

The amplitude data are then transformed from the offset-time domain tothe time-slowness domain using a high resolution Radon transformation.That is, the Radon transformation is performed according to an index jof the slowness set and a sampling variable Δp; wherein

${j = \frac{p_{\max} - p_{\min} + {1\mspace{14mu}{\mu sec}\text{/}m}}{\Delta\; p}},$Δp is from about 0.5 to about 4.0 μsec/m, p_(max) is a predeterminedmaximum slowness, and p_(min) is a predetermined minimum slowness.Preferably, an offset weighting factor x^(n) is applied to the amplitudedata, wherein 0<n<1.

Such Radon transformations include the following continuous transformequation, and discrete versions thereof that approximate the continuoustransform equation:

u(p, τ) = ∫_(−∞)^(∞)𝕕x∫_(−∞)^(∞)𝕕t d(x, t)x^(n)δ[f(t, x, τ, p)]

Radon transformations based on linear slant stack, parabolic, and othernon-hyperbolic kinematic travel time trajectories may be used, but thosebased on hyperbolic Radon kinematic trajectories are preferred. Suitablehyperbolic Radon transformations include the following continuoustransform equation, and discrete versions thereof that approximate thecontinuous transform equation:

${u\left( {p,\tau} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}x}\; x^{n}{d\left( {x,\sqrt{\tau^{2} + {p^{2}x^{2}}}} \right)}}}$Preferably, the Radon transformation is applied within defined slownesslimits p_(min) and p_(max), where p_(min) is a predetermined minimumslowness and p_(max) is a predetermined maximum slowness.

A corrective filter is then applied to the transformed data to enhancethe primary reflection signal content of the data and to eliminateunwanted noise events. Preferably, the corrective filter is a timevariant, high-low filter, and it determined by reference to the velocityfunction derived from performing a semblance analysis or a pre-stacktime migration analysis on the amplitude data.

After filtering, the enhanced signal content is inverse transformed fromthe time-slowness domain back to the offset-time domain using an inverseRadon transformation, and, if necessary, an inverse of the offsetweighting factor p^(n) is applied to the inverse transformed data,wherein 0<n<1. Such Radon transformations include the followingcontinuous inverse transform equation, and discrete versions thereofthat approximate the continuous inverse transform equation:

d(x, t) = ∫_(−∞)^(∞)𝕕p∫_(−∞)^(∞)𝕕τ p^(n)ρ(τ) * u(p, τ)δ[g(t, x, τ, p)]

Suitable hyperbolic inverse Radon transformations include the followingcontinuous inverse transform equation, and discrete versions thereofthat approximate the continuous inverse transform equation:

${d\left( {x,t} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}p}\; p^{n}{\rho(\tau)}*{u\left( {p,\sqrt{t^{2} - {p^{2}x^{2}}}} \right)}}}$

The signal amplitude data for the enhanced signal content are therebyrestored. The processed and filtered data may then be subject to furtherprocessing by which inferences about the subsurface geology of thesurvey area may be made.

It will be appreciated that the Radon transformations utilized in thesubject invention offer higher resolution as compared to prior art Radonfilters. Δp typically is from about 0.5 to about 4.0 μsec/m. A Δp ofabout 1.0 μsec/m is most preferably used. Slowness limit p_(max), whichis used to determine the index j of the slowness set, generally isgreater than the slowness of reflection signals from the shallowestreflective surface of interest. The minimum slowness limit, p_(min),generally is less than the slowness of reflection signals from thedeepest reflective surface of interest. Typically, the minimum andmaximum limits p_(min) and p_(max) will be set within 20% below (forminimum limits) or above (for maximum limits) of the slownesses of thepertinent reflection signals, or more preferably, within 10% or 5%thereof. The specific values for the slowness limits p_(min) and p_(max)will depend on the data recorded in the survey, but typically,therefore, j preferably is from about 125 to about 1000, most preferablyfrom about 250 to about 1000.

It will be appreciated that primarily because they utilize a highresolution Radon transformation, and avoid more complex mathematicaloperations required by prior art processes, the novel methods provideenhanced accuracy, yet require less total computation time andresources. Thus, even though hyperbolic Radon transformations heretoforehave generally been avoided because of their greater complexity relativeto linear slant stack and parabolic Radon transformations, the novelprocesses are able to more effectively utilize hyperbolic Radontransformations and take advantage of the greater accuracy they canprovide, especially when applied to common midpoint geometry gathers.

The subject invention also provides for improved methods of processingseismic data which comprise amplitude data assembled in the offset-timedomain in which primary reflection signals and noise overlap. Themethods include the step of enhancing the separation between primaryreflection signals and coherent noise by transforming the assembled datafrom the offset-time domain to the time-slowness domain.

More specifically, the assembled amplitude data are transformed from theoffset-time domain to the time-slowness domain using a Radontransformation according to an index j of the slowness set and asampling variable Δp; wherein

${j = \frac{p_{\max} - p_{\min} + {1\mspace{14mu}{\mu sec}\text{/}m}}{\Delta\; p}},$Δp is from about 0.5 to about 4.0 μsec/m, p_(max) is a predeterminedmaximum slowness, and p_(min) is a predetermined minimum slowness.Alternately, an offset weighting factor x^(n) is applied to theassembled amplitude data, wherein 0<n<1, and the amplitude data aretransformed with a Radon transformation. The assembled amplitude dataalso may be transformed with a Radon transformation applied withindefined slowness limits p_(min) and p_(max), where p_(min) is apredetermined minimum slowness and p_(max) is a predetermined maximumslowness, with a hyperbolic Radon transformation, or, as applied to datawhich have been uncorrected for normal moveout, with a hyperbolic orparabolic Radon transformation. Thus, the primary reflection signals andcoherent noise transform into different regions of the time-slownessdomain to enhance the separation therebetween. The enhanced separationis thereafter used in the processing to facilitate the ultimatediminishing of coherent noise in the data relative to primary reflectionsignals, and thereby, to make interpretation of the data more accurateand reliable.

The subject invention also provides for methods for determining astacking velocity function. The methods comprise the steps of processingand filtering the data as described above. A semblance analysis isperformed on the restored data to generate a refined semblance plot, anda velocity analysis is performed on the refined semblance plot to definea refined stacking velocity function. Preferably, an offset weightingfactor x^(n) is first applied to the restored amplitude data, wherein0<n<1.

It will be appreciated that, because the semblance and velocity analysisis performed on data that have been more accurately processed andfiltered, that the resulting stacking velocity function is moreaccurate. This enhances the accuracy of the many different seismicprocessing methods that utilize stacking velocity functions. Ultimately,the increased accuracy and efficiency of the novel processes enhancesthe accuracy of surveying underground geological features and,therefore, the likelihood of accurately locating the presence of oil andgas deposits.

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the U.S. Patent and TrademarkOffice upon request and payment of necessary fee.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the U.S. Patent and TrademarkOffice upon request and payment of the necessary fee.

FIG. 1 is a schematic diagram of a preferred embodiment of the methodsof the subject invention showing a sequence of steps for enhancing theprimary reflection signal content of seismic data and for attenuatingunwanted noise events, thereby rendering it more indicative ofsubsurface formations.

FIG. 2 is a schematic diagram of a two-dimensional seismic survey inwhich field records of seismic data are obtained at a number of seismicreceivers along a profile of interest.

FIG. 3 is a schematic diagram of a seismic survey depicting a commonmidpoint geometry gather.

FIG. 4 is a plot or display of model seismic data in the offset-timedomain of a common midpoint gather containing overlapping primaryreflections, a water-bottom reflection, and undesired multiplereflections.

FIG. 5 is a plot of the model seismic data of FIG. 4 aftertransformation to the time-slowness domain using a novel highresolution, hyperbolic Radon transform of the subject invention thatshows the separation of primary and water-bottom reflections frommultiple reflections.

FIG. 6 is a plot or display in the offset-time domain of the modelseismic data of FIG. 4 after applying a filter in the time-slownessdomain and inverse transforming the filtered data that shows theretention of primary and water-bottom reflections and the elimination ofmultiple reflections.

FIG. 7 is a plot in the offset-time domain of field records of seismicdata recorded along a two-dimensional profile line in the West Texasregion of the United States and assembled in a common midpoint geometrygather.

FIG. 8 is a plot in the offset-time domain of the seismic data of FIG. 7after having been processed and filtered with a high resolution Radontransformation in accordance with the subject invention.

FIG. 9 is a plot in the offset-time domain of data processed andfiltered with a high resolution Radon transformation according to thesubject invention, including the data of FIG. 8, that have been stackedin accordance with industry practices.

FIG. 10 (prior art) is in contrast to FIG. 9 and shows a plot in theoffset-time domain of unfiltered, conventionally processed data,including the unprocessed data of FIG. 7, that have been stacked inaccordance with industry practices.

FIG. 11 is a plot in the offset-time domain of field records of seismicdata recorded along a two-dimensional profile line in a West Africanproduction area and assembled in a common midpoint geometry gather.

FIG. 12 is a semblance plot of the seismic data of FIG. 11.

FIG. 13 is a plot in the offset-time domain of the seismic data of FIG.11 after having been processed and filtered with a high resolution Radontransformation in accordance with the subject invention.

FIG. 14 is a semblance plot of the processed seismic data of FIG. 13.

FIG. 15 is a plot in the offset-time domain of data processed andfiltered with a high resolution Radon transformation according to thesubject invention, including the data of FIG. 13, that have been stackedin accordance with industry practices using a stacking velocity functiondetermined from the semblance plot of FIG. 14.

FIG. 16 (prior art) is in contrast to FIG. 15 and shows a plot in theoffset-time domain of unfiltered, conventionally processed data,including the unprocessed data of FIG. 11, that have been stacked inaccordance with industry practices.

FIG. 17 is a plot in the offset-time domain of field records of seismicdata recorded along a two-dimensional profile line in the Viosca Knollregion of the United States and assembled in a common midpoint geometrygather.

FIG. 18 is a semblance plot of the seismic data of FIG. 17.

FIG. 19 is a plot in the offset-time domain of the seismic data of FIG.17 after having been processed and filtered with a high resolution Radontransformation in accordance with the subject invention.

FIG. 20 is a semblance plot of the processed seismic data of FIG. 19.

FIG. 21 is a plot in the offset-time domain of data processed andfiltered with a high resolution Radon transformation according to thesubject invention, including the data of FIG. 19, that have been stackedin accordance with industry practices using a stacking velocity functiondetermined from the semblance plot of FIG. 20.

FIG. 21A is an enlargement of the area shown in box 21A of FIG. 21.

FIG. 22 (prior art) is in contrast to FIG. 21 and shows a plot in theoffset-time domain of data, including the data of FIG. 17, that wasprocessed and filtered in accordance with conventional parabolic Radonmethods and then stacked in accordance with industry practices.

FIG. 22A (prior art) is an enlargement of the area shown in box 22A ofFIG. 22.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The subject invention is directed to improved methods for processingseismic data to remove unwanted noise from meaningful reflection signalsand for more accurately determining the stacking velocity function for aset of seismic data.

Obtaining and Preparing Seismic Data for Processing

More particularly, the novel methods comprise the step of obtainingfield records of seismic data detected at a number of seismic receiversin an area of interest. The seismic data comprise signal amplitude datarecorded over time and contains both primary reflection signals andunwanted noise events.

By way of example, a preferred embodiment of the methods of the subjectinvention is shown in the flow chart of FIG. 1. As shown therein in step1, seismic amplitude data are recorded in the offset-time domain. Forexample, such data may be generated by a seismic survey shownschematically in FIG. 2.

The seismic survey shown in FIG. 2 is a two-dimensional survey inoffset-time (X-T) space along a seismic line of profile L. A number ofseismic sources S_(n) and receivers R_(n) are laid out over a landsurface G along profile L. The seismic sources S_(n) are detonated in apredetermined sequence. As they are discharged, seismic energy isreleased as energy waves. The seismic energy waves or “signals” traveldownward through the earth where they encounter subsurface geologicalformations, such as formations F₁ and F₂ shown schematically in FIG. 2.As they do, a portion of the signal is reflected back upwardly to thereceivers R_(n). The paths of such primary reflection signals from S₁ tothe receivers R_(n) are shown in FIG. 2.

The receivers R_(n) sense the amplitude of incoming signals anddigitally record the amplitude data over time for subsequent processing.Those amplitude data recordations are referred to as traces. It will beappreciated that the traces recorded by receivers R_(n) include bothprimary reflection signals of interest, such as those shown in FIG. 2,and unwanted noise events.

It also should be understood that the seismic survey depictedschematically in FIG. 2 is a simplified one presented for illustrativepurposes. Actual surveys typically include a considerably larger numberof sources and receivers. Further, the survey may taken on land or overa body of water. The seismic sources usually are dynamite charges if thesurvey is being done on land, and geophones are used. Air guns aretypically used for marine surveys along with hydrophones. The survey mayalso be a three-dimensional survey over a surface area of interestrather than a two-dimensional survey along a profile as shown.

In accordance with the subject invention, the seismic data are assembledinto common geometry gathers in an offset-time domain. For example, instep 2 of the preferred method of FIG. 1, the seismic amplitude data areassembled in the offset-time domain as a common midpoint gather. Thatis, as shown schematically in FIG. 3, midpoint CMP is located halfwaybetween source s₁ and receiver r₁. Source s₂ and receiver r₂ share thesame midpoint CMP, as do all pairs of sources s_(n) and receivers r_(n)in the survey. Thus, it will be appreciated that all source s_(n) andreceiver r_(n) pairs are measuring single points on subsurfaceformations F_(n). The traces from each receiver r_(n) in those pairs arethen assembled or “gathered” for processing.

It will be appreciated, however, that other types of gathers are knownby workers in the art and may be used in the subject invention. Forexample, the seismic data may be assembled into common source, commonreceiver, and common offset gathers and subsequently processed toenhance signal content and attenuate noise.

It also will be appreciated that the field data may be processed byother methods for other purposes before being processed in accordancewith the subject invention as shown in step 3 of FIG. 1. Theappropriateness of first subjecting the data to amplitude balancing orother conventional pre-processing, such as spherical divergencecorrection and absorption compensation, will depend on various geologic,geophysical, and other conditions well known to workers in the art. Themethods of the subject invention may be applied to raw, unprocessed dataor to data preprocessed by any number of well-known methods. As willbecome apparent from the discussion that follows, however, it ispreferred that the data not be NMO corrected, or if preprocessed by amethod that relies on NMO correction, that the NMO correction bereversed, prior to transformation of the data. Otherwise, it is notpractical to design and apply high-low filters or to utilize limitedRadon transformations as described below.

Transformation of Data

Once the data are assembled, the methods of the subject inventionfurther comprise the step of transforming the offset weighted amplitudedata from the offset-time domain to the time-slowness domain using ahigh resolution Radon transformation. Specifically, the transformationis performed according to an index j of the slowness set and a samplingvariable Δp, wherein

${j = \frac{p_{\max} - p_{\min} + {1\mspace{14mu}{\mu sec}\text{/}m}}{\Delta\; p}},$Δp is from about 0.5 to about 4.0 μsec/m, p_(max) is a predeterminedmaximum slowness, and p_(min) is a predetermined minimum slowness. Δppreferably is from about 0.5 to about 3.0 μsec/m, and more preferably,is from about 0.5 to about 2.0 μsec/m. A Δp of about 1.0 μsec/m is mostpreferably used.

Preferably, an offset weighting factor x^(n) is applied to the amplitudedata to equalize amplitudes in the seismic data across offset values andto emphasize normal amplitude moveout differences between desiredreflection signals and unwanted noise, wherein 0<n<1. It also ispreferred that the Radon transformation be applied within definedslowness limits p_(min) and p_(max), where p_(min) is a predeterminedminimum slowness and p_(max) is a predetermined maximum slowness. Bylimiting the transformation in that manner, it is more efficient andeffective. The slowness limits p_(min) and p_(max) preferably aredetermined by reference to a velocity function derived by performing asemblance analysis or a pre-stack time migration analysis on theamplitude data.

For example, in step 4 of the exemplified method of FIG. 1, an offsetweighting factor is applied to the data that were assembled in step 2. Asemblance analysis is performed on the offset weighted amplitude data togenerate a semblance plot, as shown in step 6. Then, in step 17 thesemblance plot is used to define the slowness limits p_(min) and p_(max)that will be applied to the transformation. The offset weighted data arethen transformed with a high resolution hyperbolic Radon transformationaccording to the transform limits in step 5. It will be appreciated,however, that the Radon transformation of the offset weighted data maybe may be based on linear slant stack, parabolic, or othernon-hyperbolic kinematic travel time trajectories.

The slowness transformation limits p_(min) and p_(max) are defined byreference to the velocity function for primary reflection signals asdetermined, for example, through the semblance analysis described above.High and low transformation limits, i.e., a maximum velocity (v_(max))and minimum velocity (v_(min)), are defined on either side of thevelocity function. The velocity limits then are converted into slownesslimits p_(min) and p_(max) which will limit the slowness domain for thetransformation of the data from the offset-time domain to the tau-Pdomain, where p_(min)=1/v_(max) and p_(max)=1/v_(min).

In general, p_(max) is greater than the slowness of reflection signalsfrom the shallowest reflective surface of interest. In marine surveys,however, it typically is desirable to record the water bottom depth.Thus, p_(max) may be somewhat greater, i.e., greater than the slownessof reflective signals through water in the area of interest. The lowertransformation limit, p_(min), generally is less than the slowness ofreflection signals from the deepest reflective surface of interest.Typically, the upper and lower transformation limits p_(min) and p_(max)will be set within 20% below (for lower limits) or above (for upperlimits) of the slownesses of the pertinent reflection signals, or morepreferably, within 10% or 5% thereof. While the specific values willdepend on the data recorded in the survey, therefore, p_(min) generallywill be less than about 165 μsec/m and, even more commonly, less thanabout 185 μsec/m. Similarly, p_(max) generally will be greater thanabout 690 μsec/m, and even more commonly, greater than about 655 μsec/mfor marine surveys. For land surveys, p_(max) generally will be greaterthan about 3,125 μsec/m, and even more commonly, greater than about 500μsec/m.

When the semblance analysis is performed on amplitude data that havebeen offset weighted, as described above, appropriate slowness limitsp_(min) and p_(max) may be more accurately determined and, therefore, amore efficient and effective transformation may be defined. It will beappreciated, however, that the semblance analysis may be performed ondata that have not been offset weighted. Significant improvements incomputational efficiency still will be achieved by the subjectinvention.

The use and advantages of limited Radon transformations are morecompletely disclosed in the application of John M. Robinson, Rule 47(b)Applicant, entitled “Removal of Noise From Seismic Data Using LimitedRadon Transformations”, U.S. Ser. No. 10/238,212, filed Sep. 10, 2002,now U.S. Pat. No. 6,735,528, the disclosure of which is herebyincorporated by reference. It will be appreciated, however, that bylimiting the transformation, novel methods usually will be made moreefficient, by reducing the amount of data transformed, and moreeffective, by decimating the noise signals that fall outside thoselimits. It is not necessary, however, that effective slowness limits,either lower limits p_(min) or upper limits p_(max), be placed on thetransformation. Significant benefits still will be obtained bypracticing the subject invention as described and claimed herein.

A general mathematical formulation utilizing the offset weighting factorand encompassing the linear, parabolic, and hyperbolic forward Radontransforms is as follows:

$\begin{matrix}{{{u\left( {p,\tau} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}x}{\int_{- \infty}^{\infty}{{\mathbb{d}t}\;{d\left( {x,t} \right)}x^{n}{\delta\left\lbrack {f\left( {t,x,\tau,p} \right)} \right\rbrack}}}}}}\left( {{forward}\mspace{14mu}{transformation}} \right)} & (1)\end{matrix}$where

-   -   u(p,τ)=transform coefficient at slowness p and zero-offset time    -   τd(x,t)=measured seismogram at offset x and two-way time t    -   x^(n)=offset weighting factor (0<n<1)    -   δ=Dirac delta function    -   ƒ(t,x,τ,p)=forward transform function

The forward transform function for hyperbolic trajectories is asfollows:ƒ(t,x,τ,p)=t−√{square root over (τ² +p ² x ²)}Thus, when t=√{square root over (τ²+p²x²)}, the forward hyperbolic Radontransformation equation becomes

${u\left( {p,\tau} \right)} = {\int_{- \infty}^{\infty}\ {{\mathbb{d}{xx}^{n}}{\mathbb{d}\left( {x,\sqrt{\tau^{2} + {p^{2}x^{2}}}} \right)}}}$

The forward transform function for linear slant stack kinematictrajectories is as follows:ƒ(t,x,τ,p)=t−τ−pxThus, when t=τ+px, the forward linear slant stack Radon transformationequation becomes

u(p, τ) = ∫_(−∞)^(∞) 𝕕xx^(n)𝕕(x, τ + px)

The forward transform function for parabolic trajectories is as follows:ƒ(t,x,τ,p)=t−τ−px ²Thus, when t=τ+px², the forward parabolic Radon transformation equationbecomes

u(p, τ) = ∫_(−∞)^(∞) 𝕕xx^(n)𝕕(x, τ + px²)

As with conventional Radon transformations, the function ƒ(t,x,τ,p)allows kinematic travel time trajectories to include anisotropy, P-Sconverted waves, wave-field separations, and other applications ofcurrent industry that are used to refine the analysis consistent withconditions present in the survey area. Although hyperbolic travel timetrajectories represent more accurately reflection events for commonmidpoint gathers in many formations, hyperbolic Radon transformations todate have not been widely used. Together with other calculationsnecessary to practice prior art processes, the computational intensityof hyperbolic Radon transforms tended to make such processing moreexpensive and less accurate. Hyperbolic Radon transformations, however,are preferred in the context of the subject invention because thecomputational efficiency of the novel processes allows them to takeadvantage of the higher degree of accuracy provided by hyperbolic traveltime trajectories.

It will be noted, however, that the novel Radon transformations setforth above in Equation 1 differ from conventional Radon transformationsin the application of an offset weighting factor x^(n), where x isoffset. The offset weighting factor emphasizes amplitude differencesthat exist at increasing offset, i.e., normal amplitude moveout itdifferences between desired primary reflections and multiples, linear,and other noise whose time trajectories do not fit a defined kinematicfunction. Since the offset is weighted by a factor n that is positive,larger offsets receive preferentially larger weights. The power n isgreater than zero, but less than 1. Preferably, n is approximately 0.5since amplitudes seem to be preserved better at that value. While thepower n appears to be robust and insensitive, it probably is datadependent to some degree.

While the application of an offset weighting factor as described aboveis preferred, it will be appreciated that other methods of amplitudebalancing may be used in the methods of the subject invention. Forexample, automatic gain control (AGC) operators may be applied. A gaincontrol operator may be applied where the gain control operator is theinverse of the trace envelope for each trace as disclosed in U.S. Pat.No. 5,189,644 to Lawrence C. Wood and entitled “Removal of AmplitudeAliasing Effect From Seismic Data”.

As will be appreciated by workers in the art, execution of thetransformation equations discussed above involve numerous calculationsand, as a practical matter, must be executed by computers if they are tobe applied to the processing of data as extensive as that contained in atypical seismic survey. Accordingly, the transformation equations, whichare expressed above in a continuous form, preferably are translated intodiscrete transformation equations which approximate the solutionsprovided by continuous transformation equations and can be encoded intoand executed by computers.

For example, assume a seismogram d(x,t) contains 2L+1 traces each havingN time samples, i.e.,l=0, ±1, . . . , ±L andk=1, . . . , Nand thatx_(−L)<x_(−L+1)< . . . <x_(L−1)<x_(L)A discrete general transform equation approximating the continuousgeneral transform Equation 1 set forth above, therefore, may be derivedas set forth below:

$\begin{matrix}{{u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}\;{\sum\limits_{k = 1}^{N}\;{{\mathbb{d}\left( {x_{l},t_{k}} \right)}x_{l}^{n}{\delta\left\lbrack {f\left( {t_{k},x_{l},\tau,p} \right)} \right\rbrack}\Delta\; x_{l}\Delta\; t_{k}}}}} & (2)\end{matrix}$where

${{\Delta\; x_{l}} = {{\frac{x_{l + 1} - x_{l - 1}}{2}{for}\mspace{14mu} l} = 0}},{\pm 1},\ldots\mspace{14mu},{\pm \left( {L - 1} \right)}$Δx _(L) =x _(L) −x _(L−1)Δx _(−L) =x _(−L+1) −x _(−L)

${{\Delta\; t_{k}} = {{\frac{t_{k + 1} - t_{k - 1}}{2}\mspace{14mu}{for}\mspace{14mu} k} = 2}},\ldots\mspace{14mu},{N - 1}$Δt ₁ =t ₂ −t ₁Δt _(N) =t _(N) −t _(N−1)

By substituting the hyperbolic forward transform function set forthabove, the discrete general forward transformation Equation 2 above,when t=√{square root over (τ²+p²x²)}, may be reduced to the discretetransformation based on hyperbolic kinematic travel time trajectoriesthat is set forth below:

${u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}{x_{l}^{n}{\mathbb{d}\left( {x_{l},\sqrt{\tau^{2} + {p^{2}x_{l}^{2}}}} \right)}\Delta\; x_{l}}}$

Similarly, when t=τ+px, the discrete linear slant stack forwardtransformation derived from general Equation 2 is as follows:

${u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}{x_{l}^{n}{\mathbb{d}\left( {x_{l},{\tau + {px}_{l}}} \right)}\Delta\; x_{l}}}$

When t=τ+px², the discrete parabolic forward transformation is asfollows:

${u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}{x_{l}^{n}{\mathbb{d}\left( {x_{l},{\tau + {px}_{l}^{2}}} \right)}\Delta\; x_{l}}}$

Those skilled in the art will appreciate that the foregoing discretetransformation Equation 2 sufficiently approximates continuous Equation1, but still may be executed by computers in a relatively efficientmanner. For that reason, the foregoing equation and the specific casesderived therefrom are preferred, but it is believed that other discretetransformation equations based on Radon transformations are known, andstill others may be devised by workers of ordinary skill for use in thesubject invention.

It will be appreciated, however, that the subject invention provides forthe transformation of data with a high resolution Radon filter. That is,the transformation is performed according to an index j of the slownessset and a sampling variable Δp, wherein

${j = \frac{p_{\max} - p_{\min} + {1\mu\;\sec\text{/}m}}{\Delta\; p}},$p_(max) is a predetermined maximum slowness, and p_(min) is apredetermined minimum slowness. Thus, the novel methods provide finerresolution transformations and, therefore, better resolution in thefiltered data.

More specifically, Δp typically is from about 0.5 to about 4.0 μsec/m.Δp preferably is from about 0.5 to about 3.0 μsec/m, and morepreferably, is from about 0.5 to about 2.0 μsec/m. A p of about 1.0μsec/m is most preferably used. Slowness limits p_(min) and p_(max),which are used to determine the index j of the slowness set, aregenerally set in the same manner as the slowness limits that preferablyare applied to limit the transformation as described above. That is,p_(max) generally is greater than the slowness of reflection signalsfrom the shallowest reflective surface of interest, although in marinesurveys it may be somewhat greater, i.e., greater than the slowness ofreflective signals through water in the area of interest, so as torecord the water bottom depth. The minimum slowness limit, p_(min),generally is less than the slowness of reflection signals from thedeepest reflective surface of interest. Typically, the minimum andmaximum limits p_(min) and p_(max) will be set within 20% below (forminimum limits) or above (for maximum limits) of the slownesses of thepertinent reflection signals, or more preferably, within 10% or 5%thereof. The specific values for the slowness limits p_(min) and p_(max)will depend on the data recorded in the survey, but typically,therefore, j preferably is from about 125 to about 1000, most preferablyfrom about 250 to about 1000.

Such values are significantly finer than prior art Radon processes,where Δp is typically set at values in the range of 4-24 μsec/m. Also,the index j of the slowness set usually is equal to the fold of theoffset data, which typically ranges from about 20 to about 120. Thenovel processes, therefore, provide a much higher resolutiontransformation and, to that extent, increased accuracy in the filteringof the data.

It also will be appreciated that this increased resolution and accuracymay be achieved without reliance on computationally intensive processingsteps, such as trace interpolation, LMS analysis in the transformation,or NMO correction of the data prior to transformation, which alsorequires a LMS analysis. Moreover, it is possible to avoid the loss ofresolution caused by the use of coarse sampling variables in NMOcorrecting, i.e., Δt values in the range of 20-40 milliseconds and Δvvalues of from about 15 to about 120 m/sec.

Filtering of Data

The methods of the subject invention further comprise the step ofapplying a corrective filter to enhance the primary reflection signalcontent of the data and to eliminate unwanted noise events. Preferably,the corrective filter is a high-low filter, most preferably, a timevariant, high-low filter. The corrective filter preferably is determinedby performing a semblance analysis on the amplitude data to generate asemblance plot and performing a velocity analysis on the semblance plotto define a stacking velocity function and the corrective filter. Thecorrective filter also may be determined by performing a pre-stack timemigration analysis on the amplitude data to define a velocity functionand the corrective filter.

For example, in step 6 of the preferred method of FIG. 1, a semblanceanalysis is performed on the offset weighted amplitude data to generatea semblance plot. Then, in step 7 a velocity analysis is performed onthe semblance plot to define a stacking velocity function and a timevariant, high-low corrective filter. The corrective filter then isapplied to the transformed data as shown in step 8.

When the semblance analysis is performed on amplitude data that havebeen offset weighted, as described above, the resulting velocityanalysis will be more accurate and, therefore, a more effective filtermay be defined. It will be appreciated, however, that the semblanceanalysis may be performed on data that have not been offset weighted.Significant improvements in accuracy and computational efficiency stillwill be achieved by the subject invention.

The high-low filter is defined by reference to the velocity function forprimary reflection signals, as determined, for example, through thesemblance analysis described above. The stacking velocity function,[t₀,v_(s)(t₀)], describes the velocity of primary reflection signals asa function of t₀. High and low filter limits, i.e., a maximum velocity,v_(max)(t₀), and a minimum velocity, v_(min)(t₀), are defined on eitherside of the velocity function, for example, within given percentages ofthe velocity function. In such cases, the velocity pass region at aselected time t₀ corresponds to v_(s)(1−r₁)≦v≦v_(s)(1+r₂), where r₁ andr₂ are percentages expressed as decimals. The velocity function willtransform to a slowness function, [τ,p(τ)], in the tau-P domain.Similarly, the velocity pass region will map into a slowness passregion, namely:

$\frac{1}{v_{s}\left( {1 + r_{2}} \right)} \leq p \leq \frac{1}{v_{s}\left( {1 - r_{1}} \right)}$for application to the transformed data in the tau-P domain. Theslowness limits typically are set within ±15%, preferably within ±10%,and most preferably within ±5% of the velocity function, and r₁ and r₂may be set accordingly.

Since the velocity function for the primary reflection signals is timevariant, the filter limits preferably are time variant as well. In thatmanner, the filter may more closely fit the reflection's velocityfunction and, therefore, more effectively isolate primary reflectionsignals and water bottom reflection signals and remove multiplereflection signals and other noise. Moreover, since the strongest noisesignals typically occur at velocities slower than primary reflectionsignals, it is preferable that v_(min) lower limit more closelyapproximate the velocity function of the primary reflection signals.

For such reasons and others as will be appreciated by workers in theart, time variant, high-low filters generally are more effective inremoving noise and, thus, generally are preferred for use in the subjectinvention. The use and advantages of time variant, high low tau-Pfilters are more completely disclosed in the application of John M.Robinson, Rule 47(b) Applicant, entitled “Removal of Noise From SeismicData Using Improved Tau-P Filters”, U.S. Ser. No. 10/238,461, filed Sep.10, 2002, now U.S. Pat. No. 6,721,662, the disclosure of which is herebyincorporated by reference.

Though they usually will not be as effective as when the filter limitsare time variable, however, high-low filters may be used where one orboth of the filter limits are time invariant. Mute filters based on adefined p_(mute) also may be used, and, if the data have been NMOcorrected prior to transformation, generally will be required. Othertau-P filters are known by workers in the art and may be used.

Finally, it will be appreciated that while the subject inventioncontemplates the application of a filter to the transformed data in thetau-P domain, additional filters may be applied to the data in thetime-space domain before or after application of the Radontransformations, or in other domains by other filtration methods, ifdesired or appropriate.

The design of a corrective filter is guided by the following principles.Seismic waves can be analyzed in terms of plane wave components whosearrival times in X-T space are linear. Seismic reflection times areapproximately hyperbolic as a function of group offset X in thefollowing relations:

$t_{x}^{2} = {t_{o}^{2} + \frac{x^{2}}{v_{s}^{2}}}$where

-   -   x is the group offset,    -   t_(x) is the reflection time at offset x;    -   t_(o) is the reflection time at zero offset; and    -   v_(s) is the stacking velocity.

The apparent surface velocity of a reflection at any given offsetrelates to the slope of the hyperbola at that offset. Those slopes, andthus, the apparent velocities of reflections are bounded betweeninfinite velocity at zero offset and the stacking velocity at infiniteoffset. Noise components, however, range from infinite apparent velocityto about one-half the speed of sound in air.

Impulse responses for each Radon transform can be readily calculated bysetting g(t,x,τ₁,p₁)=0 or ƒ(t₁,x₁,τ,p)=0 in the respective domains, forexample, by substituting a single point sample such as (x₁,t₁) or(τ₁,p₁) in each domain. Substitutingd(x ₁ ,t ₁)=Aδ(x−x ₁)δ(t−t ₁)into the linear slant stack equation yieldsu(p,τ)=Aδ(t ₁ −τ−px ₁).Thus it shows that a point of amplitude A at (x₁,t₁) maps into thestraight lineτ+px ₁ =t ₁.

On the other hand, substituting (p₁,τ₁) of amplitude B yieldsu(p,τ)=Bδ(τ₁ −t+p ₁ x)showing that a point (p₁,τ₁) in tau-P space maps into the lineτ₁ =t−p ₁ x.

In a similar manner it can be shown that a hyperbola

$t^{2} = {t_{1}^{2} + \frac{x^{2}}{v^{2}}}$maps into the point(τ,p)=(t ₁,1/v),and vice-versa.

In the practice of the subject invention a point (x₁,t₁) maps into theellipse

$1 = {\frac{\tau^{2}}{t_{1}^{2}} + \frac{p^{2}x_{1}^{2}}{t_{1}^{2}}}$(X-T domain impulse response) under the hyperbolic Radon transform.Thus, hyperbolas representing signals map into points.

In addition straight linest=mxrepresenting organized source-generated noise also map into points(τ,p)=(o,m)in the tau-P domain. These properties form the basis for designing tau-Pfilters that pass signal and reject noise in all common geometrydomains.

In summary then, in a common midpoint gather prior to NMO correction,the present invention preserves the points in tau-P space correspondingto the hyperbolas described by the stacking velocity function (t₁,v),where v is the dip corrected stacking velocity. A suitable error oftolerance such as 5-20% is allowed for the stacking velocity function.Similar procedures exist for each of the other gather geometries.

The signal's characterization is defined by the Canonical equations thatare used to design two and three dimensional filters in each commongeometry gather. Filters designed to pass signal in the tau-P domain forthe common geometry gathers for both two-and-three dimensional seismicdata are derived from the following basic considerations. In the X-Tdomain the kinematic travel time for a primary reflection event's signalin a common midpoint (“CMP”) gather is given by the previously describedhyperbolic relationship

$t_{x}^{2} = {t_{o}^{2} + \frac{x^{2}}{v_{s}^{2}}}$Under the hyperbolic Radon transformation, this equation becomest _(x) ²τ² +p ² x ².

The source coordinate for two-dimensional seismic data may be denoted byI (Initiation) and the receiver coordinate by R (receiver) where τ=t₀and p=1/v_(s). The signal slowness in the tau-P domain for common sourcedata is given by

$\left. \frac{\partial t_{x}}{\partial R} \right|_{I = I_{o}} = {\frac{{\partial t_{x}}{\partial C}}{{\partial C}{\partial R}} + \frac{{\partial t}{\partial x}}{{\partial x}{\partial R}}}$where the CMP coordinate (C) and offset (x) are as follows:

$C = \frac{I + R}{2}$ x = R − IDefining common source slowness (p_(R)), common offset slowness (p_(C)),and common midpoint slowness (p_(x)) as follows:

$p_{R} = {\left. \frac{\partial t_{x}}{\partial R} \middle| {}_{I}p_{C} \right. = {\left. \frac{\partial t_{x}}{\partial C} \middle| {}_{x}p_{x} \right. = \left. \frac{\partial t_{x}}{\partial x} \right|_{C}}}$we obtain a general (canonical) relationship for signal slowness in thetau-P domain:

$p_{R} = {{\frac{1}{2}p_{C}} + p_{x}}$Common offset slowness p_(C) is given by

$p_{C} = {\frac{{\partial t_{x}}{\partial t_{o}}}{{\partial t_{o}}{\partial C}} = {\frac{\partial t_{x}}{\partial t_{o}}p_{o}}}$where p₀ is the slowness or “dip” on the CMP section as given by

$p_{o} = \frac{\partial t_{o}}{\partial C}$Since

${t_{x}^{2} = {t_{o}^{2} + \frac{x^{2}}{v_{s}^{2}}}},$we obtain

${\frac{\partial t_{x}}{\partial t_{o}} = {\frac{t_{o}}{t_{x}} - \frac{x^{2}a}{t_{x}v_{s}^{3}}}},$where acceleration (a)

$a = {\frac{\partial v_{s}}{\partial t_{o}}.}$Thus, we obtain that common offset slowness (p_(C))

$p_{C} = {p_{o}\frac{t_{o}}{t_{x}}{\left( {1 - \frac{{ax}^{2}}{t_{o}v_{s}^{3}}} \right).}}$Under a hyperbolic Radon transformation whereτ=t_(o)t _(x)=√{square root over (τ² +p ² x ²)},we determine that the common midpoint slowness (p_(x)), common sourceslowness (p_(R)), common receiver slowness (p_(I)), and common offsetslowness (p_(C)) may be defined as follows:

$\begin{matrix}{p_{x} = \frac{x}{v_{s}^{2}\sqrt{\tau^{2} + {p^{2}x^{2}}}}} \\{p_{R} = {{\frac{1}{2}p_{C}} + p_{x}}} \\{p_{I} = {{\frac{1}{2}p_{C}} - p_{x}}} \\{p_{C} = {p_{o}\frac{\tau}{\sqrt{\tau^{2} + {p^{2}x^{2}}}}\mspace{11mu}\left( {1 - \frac{{ax}^{2}}{t_{o}v_{s}^{3}}} \right)}}\end{matrix}$

In common source, receiver and offset gathers, the pass regions orfilters in tau-P space are defined as those slowness (p's) not exceedingspecified percentages of slownesses p_(R), p_(I), and p_(C),respectively:

$p \leq {\left( \frac{1}{1 + r_{3}} \right)p_{R}\mspace{20mu}\left( {{common}\mspace{14mu}{source}} \right)}$$p \leq {\left( \frac{1}{1 + r_{4}} \right)p_{I}\mspace{20mu}\left( {{common}\mspace{14mu}{receiver}} \right)}$$p \leq {\left( \frac{1}{1 + r_{5}} \right)p_{C}\mspace{20mu}\left( {{common}\mspace{14mu}{offset}} \right)}$where r₃, r₄, and r₅ are percentages expressed as decimals.Slownesses p_(R), p_(I), and p_(C) are expressed, respectively, asfollows:

$p_{R} = {{\frac{1}{2}p_{C}} + {p_{x}\mspace{20mu}\left( {{common}\mspace{14mu}{source}} \right)}}$$p_{I} = {{\frac{1}{2}p_{C}} - {p_{x}\mspace{20mu}\left( {{common}\mspace{14mu}{receiver}} \right)}}$$p_{C} = {\frac{t_{o}}{t_{x}}{p_{x}\left( {1 - \frac{{ax}^{2}}{t_{o}v_{s}^{3}}} \right)}\mspace{20mu}\left( {{common}\mspace{14mu}{offset}} \right)}$where $p_{x} = \frac{x}{t_{x}v_{s}^{2}}$$a = {\frac{\partial v_{s}}{\partial t_{o}}\mspace{14mu}\left( {{velocity}\mspace{14mu}{acceleration}} \right)}$$p_{o} = {\frac{\partial t_{o}}{\partial C}\mspace{14mu}\left( {{dip}\mspace{14mu}{on}\mspace{14mu}{CMP}\mspace{14mu}{stack}} \right)}$$t_{X} = \sqrt{t_{o}^{2} + \frac{x^{2}}{v_{s}^{2}}}$ τ = t_(o)The value for offset x in these equations is the worse case maximumoffset determined by the mute function.

Inverse Transforming the Data

After the corrective filter is applied, the methods of the subjectinvention further comprise the steps of inverse transforming theenhanced signal content from the time-slowness domain back to theoffset-time domain using an inverse Radon transformation. If, as ispreferable, an offset weighting factor x^(n) was applied to thetransformed data, an inverse of the offset weighting factor p^(n) isapplied to the inverse transformed data. Similarly, if other amplitudebalancing operators, such as an AGC operator or an operator based ontrace envelopes, were applied, an inverse of the amplitude balancingoperator is applied. The signal amplitude data are thereby restored forthe enhanced signal content.

For example, in step 9 of the method of FIG. 1, an inverse hyperbolicRadon transformation is used to inverse transform the enhanced signalfrom the time-slowness domain back to the offset-time domain. An inverseof the offset weighting factor p^(n) then is applied to the inversetransformed data, as shown in step 10. The signal amplitude data for theenhanced signal content are thereby restored.

A general mathematical formulation utilizing the inverse offsetweighting factor and encompassing the linear, parabolic, and hyperbolicinverse Radon transforms is as follows:

$\begin{matrix}{{d\left( {x,t} \right)} = {\int_{- \infty}^{\infty}\mspace{7mu}{{\mathbb{d}p}{\int_{- \infty}^{\infty}\mspace{7mu}{{\mathbb{d}\tau}\; p^{n}{\rho(\tau)}*{u\left( {p,\tau} \right)}{\delta\left\lbrack {g\left( {t,x,\tau,p} \right)} \right\rbrack}\mspace{14mu}\left( {{inverse}\mspace{14mu}{transformation}} \right)}}}}} & (3)\end{matrix}$where

-   -   u(p,τ)=transform coefficient at slowness p and zero-offset time        τ    -   d(x,t)=measured seismogram at offset x and two-way time t    -   p^(n)=inverse offset weighting factor (0<n<1)    -   ρ(τ)*=convolution of rho filter    -   δ=Dirac delta function    -   g(t,x,τ,p)=inverse transform function

The inverse transform function for hyperbolic trajectories is asfollows:g(t,x,τ,p)=τ−√{square root over (t ² −p ² x ²)}Thus, when τ=√{square root over (t²−p²x²)}, the inverse hyperbolic Radontransformation equation becomes

${d\left( {x,t} \right)} = {\int_{- \infty}^{\infty}\mspace{7mu}{{\mathbb{d}{pp}^{n}}{\rho(\tau)}*{u\left( {p,\sqrt{t^{2} - {p^{2}x^{2}}}} \right)}}}$

The inverse transform function for linear slant stack trajectories is asfollows:g(t,x,τ,p)=τ−t+pxThus, when τ=t−px, the inverse linear slant stack Radon transformationequation becomes

d(x, t) = ∫_(−∞)^(∞)𝕕pp^(n)ρ(τ) * u(p, t − px)

The inverse transform function for parabolic trajectories is as follows:g(t,x,τ,p)=τ−t+px ²Thus, when τ=t−px², the inverse parabolic Radon transformation equationbecomes

d(x, t) = ∫_(−∞)^(∞)𝕕pp^(n)ρ(τ) * u(p, t − px²)

As with the forward transform function ƒ(t,x,τ,p) in conventional Radontransformations, the inverse travel-time function g(t,x,τ,p) allowskinematic travel time trajectories to include anisotropy, P-S convertedwaves, wave-field separations, and other applications of currentindustry that are used to refine the analysis consistent with conditionspresent in the survey area. It will be noted, however, that the novelinverse Radon transformations set forth above in Equation 3 differ fromconventional inverse Radon transformations in the application of aninverse offset weighting factor p^(n), where p is slowness.

The inverse offset weighting factor restores the original amplitude datawhich now contain enhanced primary reflection signals and attenuatednoise signals. Accordingly, smaller offsets receive preferentiallylarger weights since they received preferentially less weighting priorto filtering. The power n is greater than zero, but less than 1.Preferably, n is approximately 0.5 because amplitudes seem to bepreserved better at that value. While the power n appears to be robustand insensitive, it probably is data dependent to some degree.

As discussed above relative to the forward transformations, thecontinuous inverse transformations set forth above preferably aretranslated into discrete transformation equations which approximate thesolutions provided by continuous transformation equations and can beencoded into and executed by computers.

For example, assume a transform coefficient u(p,τ) contains 2J+1discrete values of the parameter p and M discrete values of theparameter τ, i.e.,j=0, ±1, . . . , ±J andm=1, . . . , Mand thatp_(−J)<p_(−J+1)< . . . <p_(J−1)<p_(J)A discrete general transform equation approximating the continuousgeneral transform Equation 3 set forth above, therefore, may be derivedas set forth below:

$\begin{matrix}{{d\left( {x,t} \right)} = {\sum\limits_{J = {- J}}^{J}{\sum\limits_{m = 1}^{M}{{u\left( {p_{j},\tau_{m}} \right)}p_{j}^{n}{\rho(\tau)}*{\delta\left\lbrack {g\left( {t,x,\tau_{m},p_{j}} \right)} \right\rbrack}\Delta\; p_{j}\Delta\;\tau_{m}}}}} & (4)\end{matrix}$where

${{\Delta\; p_{j}} = {{\frac{p_{j + 1} - p_{j - 1}}{2}\mspace{14mu}{for}\mspace{14mu} j} = 0}},{\pm 1},\ldots\mspace{11mu},{\pm \left( {J - 1} \right)}$Δp _(J) =p _(J) −p _(J−1)Δp _(−J) =p _(−J+1) −p _(−J)

${{\Delta\;\tau_{m}} = {{\frac{\tau_{m + 1} - \tau_{m - 1}}{2}\mspace{14mu}{for}\mspace{14mu} m} = 2}},\ldots\mspace{11mu},{M - 1}$Δτ₁=τ₂−τ₁Δτ_(M)=τ_(M)−τ_(M−1)

By substituting the hyperbolic inverse transform function set forthabove, the discrete general inverse transformation Equation 4 above,when τ=√{square root over (t²−p²x²)}, may be reduced to the discreteinverse transformation based on hyperbolic kinematic travel timetrajectories that is set forth below:

${d\left( {x,t} \right)} = {\sum\limits_{j = {- J}}^{J}{p_{j}^{n}{\rho(\tau)}*{u\left( {p_{j},\sqrt{t^{2} - {p_{j}^{2}x^{2}}}}\; \right)}\Delta\; p_{j}}}$

Similarly, when τ=t−px, the discrete linear slant stack inversetransformation derived from the general Equation 4 is as follows:

${d\left( {x,t} \right)} = {\sum\limits_{j = {- J}}^{J}{p_{j}^{n}{\rho(\tau)}*{u\left( {p_{j},{t - {p_{j}x}}} \right)}\Delta\; p_{j}}}$

When τ=t−px², the discrete parabolic inverse transformation is asfollows:

${d\left( {x,t} \right)} = {\sum\limits_{j = {- J}}^{J}{p_{j}^{n}{\rho(\tau)}*{u\left( {p_{j},{t - {p_{j}x^{2}}}} \right)}\Delta\; p_{j}}}$

Those skilled in the art will appreciate that the foregoing inversetransformation Equations 3 and 4 are not exact inverses of the forwardtransformation Equations 1 and 2. They are sufficiently accurate,however, and as compared to more exact inverse equations, they are lesscomplicated and involve fewer mathematical computations. For example,more precise inverse transformations could be formulated with Fouriertransform equations, but such equations require an extremely largenumber of computations to execute. For that reason, the foregoingequations are preferred, but it is believed that other inversetransformation equations are known, and still others may be devised byworkers of ordinary skill for use in the subject invention.

The transformations and semblance analyses described above, as will beappreciated by those skilled in the art, are performed by sampling thedata according to sampling variables Δt, Δx, Δτ, and Δp. Because thenovel methods do not require NMO correction prior to transformation or aleast mean square analysis, sampling variables may be much finer.Specifically, Δt and Δτ may be set as low as the sampling rate at whichthe data were collected, which is typically 1 to 4 milliseconds, or evenlower if lower sampling rates are used. Δp values as small as about 0.5μsec/m may be used. Preferably, Δp is from about 0.5 to 4.0 μsec/m, morepreferably from about 0.5 to about 3.0 μsec/m, and even more preferablyfrom about 0.5 to about 2.0 μsec/m. Most preferably Δp is set at 1.0μsec/m. Since the sampling variables are finer, the calculations aremore precise. It is preferred, therefore, that the sampling variablesΔt, Δx, and Δτ be set at the corresponding sampling rates for theseismic data field records and that Δp be set at 1 μsec/m. In thatmanner, all of the data recorded by the receivers is processed. It alsois not necessary to interpolate between measured offsets.

Moreover, the increase in accuracy achieved by using finer samplingvariables in the novel processes is possible without a significantincrease in computation time or resources. Although the noveltransformation equations and offset weighting factors may be operatingon a larger number of data points, those operations require fewercomputations that prior art process. On the other hand, coarser samplingvariables more typical of prior art processes may be used in the novelprocess, and they will yield accuracy comparable to that of the priorart, but with significantly less computational time and resources.

For example, industry commonly utilizes a process known as constantvelocity stack (CVS) panels that in some respects is similar toprocesses of the subject invention. CVS panels and the subject inventionboth are implemented on common mid-point gathers prior to commonmid-point stacking and before NMO correction. Thus, the starting pointof both processes is the same.

In the processes of the subject invention, the discrete hyperbolic radontransform u(p,τ) is calculated by summing seismic trace amplitudesd(x,t) along a hyperbolic trajectoryt=√{square root over (τ² +p ² x ²)}in offset-time (x,t) space for a particular tau-p (τ,p) parameterselection. Amplitudes d(x,t) are weighted according to a power n of theoffset x, i.e., by the offset weighting factor x^(n). The summation andweighting process is described by the discrete forward transformEquation 2. In comparison, a CVS panel does not apply any offsetdependent weighting.

In the processes of the subject invention, the calculation consists offirst scanning over all desired zero-offset times τ for a givenspecification of p, and then changing the value of p and repeating thex-t summation for all desired taus (τ's). Thus, all desired values of τand p are accounted for in the summations described by Equation 2 above.

A given choice of p describes a family of hyperbolas over the commonmidpoint gather's x-t space. A fixed value of p is equivalent to fixingthe stacking velocity V_(s) since p is defined as its reciprocal, viz.,p=1/V_(s). By varying the zero-offset time τ over the desired τ-scanrange for a fixed value of p=1/V_(s), seismic amplitudes are summed inx-t space over a family of constant velocity hyperbolic trajectories.Thus, the novel Radon transforms u(p,τ) can be thought of assuccessively summing amplitudes over constant velocity hyperbolic timetrajectories that have not been NMO corrected.

In constructing a CVS panel the starting point is a common midpointgather. As in the case of the novel processes, successive scans overstacking velocity V_(s) and zero-offset time T_(o) are implemented forhyperbolic trajectories defined by the equation

$t = \sqrt{T_{o}^{2} + \frac{x^{2}}{v_{s}^{2}}}$

The equivalence of T_(o)=τ and p=1/V_(s) in defining the same familiesof hyperbolas is obvious. The difference between the novel processes andCVS is in the implementation of amplitude summations. In both cases, aV_(s)=1/p is first held constant, and the T_(o)=τ is varied. A newV_(s)=1/p is then held constant, and the T_(o)=τ scan repeated. In thismanner, the same families of hyperbolas are defined. However, an NMOcorrection is applied to all the hyperbolas defined by the V_(s)=1/pchoice prior to summing amplitudes. A CVS trace is calculated for eachconstant velocity iteration by first NMO correcting the traces in thecommon midpoint gather and then stacking (i.e., summing) the NMOcorrected traces. Thus, one CVS trace in a CVS panel is almost identicalto a u(p=constant,τ) trace in the novel processes. The difference liesin whether amplitudes are summed along hyperbolic trajectories withoutan NMO correction as in the novel process or summed (stacked) after NMOcorrection.

The novel processes and CVS differ in the interpolations required toestimate seismic amplitudes lying between sample values. In the novelprocesses amplitude interpolations occur along hyperbolic trajectoriesthat have not been NMO corrected. In the CVS procedure amplitudeinterpolation is an inherent part of the NMO correction step. The novelprocesses and CVS both sum over offset x and time t after interpolationto produce a time trace where p=1/V_(s) is held constant over timeT_(o)=τ. Assuming interpolation procedures are equivalent or very close,then a single CVS time trace computed from a particular common midpointgather with a constant V_(s)=1/p is equivalent in the novel processes toa time trace u(p,τ) where p=1/V_(s) is held constant. Thus, in actualpractice the velocity ensemble of CVS traces for a single commonmidpoint gather corresponds to a coarsely sampled transformation of thesame common midpoint gather according to the novel processes. As noted,however, the novel processes are capable of using finer samplingvariables and thereby of providing greater accuracy.

Refining of the Stacking Velocity Function

The subject invention also encompasses improved methods for determininga stacking velocity function which may be used in processing seismicdata. Such improved methods comprise the steps of performing a semblanceanalysis on the restored data to generate a second semblance plot.Though not essential, preferably an offset weighting factor x^(n), where0<n<1, is first applied to the restored data. A velocity analysis isthen performed on the second semblance plot to define a second stackingvelocity function. It will be appreciated that this second stackingvelocity, because it has been determined based on data processed inaccordance with the subject invention, more accurately reflects the truestacking velocity function and, therefore, that inferred depths andcompositions of geological formations and any other subsequentprocessing of the data that utilizes a stacking velocity function aremore accurately determined.

For example, as shown in step 11 of FIG. 1, an offset weighting factorx^(n) is applied to the data that were restored in step 10. A semblanceplot is then generated from the offset weighted data as shown in step12. The semblance plot is used to determine a stacking velocity functionin step 13 which then can be used in further processing as in step 14.Though not shown on FIG. 1, it will be appreciated that the semblanceanalysis can be performed on the inverse transformed data from step 9,thereby effectively applying an offset weighting factor since the effectof offset weighting the data prior to transformation has not beeninversed.

Display and Further Processing of Data

After the data are restored, they may be displayed for analysis, forexample, as shown in step 15 of FIG. 1. The restored data, as discussedabove, may be used to more accurately define a stacking velocityfunction. It also may subject to further processing before analysis asshown in step 16. Such processes may include pre-stack time or depthmigration, frequency-wave number filtering, other amplitude balancingmethods, removal of aliasing effects, seismic attribute analysis,spiking deconvolution, data stack processing, and other methods known toworkers in the art. The appropriateness of such further processing willdepend on various geologic, geophysical, and other conditions well knownto workers in the art.

Invariably, however, the data in the gather, after they have beenprocessed and filtered in accordance with the subject invention, will bestacked together with other data gathers in the survey that have beensimilarly processed and filtered. The stacked data are then used to makeinference about the subsurface geology in the survey area.

The methods of the subject invention preferably are implemented bycomputers and other conventional data processing equipment. Suitablesoftware for doing so may be written in accordance with the disclosureherein. Such software also may be designed to process the data byadditional methods outside the scope of, but complimentary to the novelmethods. Accordingly, it will be appreciated that suitable software willinclude a multitude of discrete commands and operations that may combineor overlap with the steps as described herein. Thus, the precisestructure or logic of the software may be varied considerably whilestill executing the novel processes.

EXAMPLES

The invention and its advantages may be further understood by referenceto the following examples. It will be appreciated, however, that theinvention is not limited thereto.

Example 1 Model Data

Seismic data were modeled in accordance with conventional considerationsand plotted in the offset-time domain as shown in FIG. 4. Such datadisplay a common midpoint gather which includes desired primaryreflections Re_(n) and undesired multiples reflections M_(n) beginningat 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 seconds. It also includes a desiredwater-bottom reflection WR at 0.5 seconds. It will be noted inparticular that the signals for the desired reflections Re_(n) and WRoverlap with those of the undesired multiple reflections M_(n).

The modeled seismic data were then transformed into the time-slownessdomain utilizing a high resolution, hyperbolic Radon transformation ofthe subject invention and an offset weighting factor x^(0.5). As shownin FIG. 5, the reflections Re_(n), WR, and M_(n) all map to concentratedpoint-like clusters in time-slowness space. Those points, however, mapto different areas of time-slowness space, and thus, the pointscorresponding to undesired multiple reflections M_(n) can be eliminatedby appropriate time-slowness filters, such as the time variant, high-lowslowness filter represented graphically by lines 20. Thus, when themodel data are inverse transformed back to the offset-time domain by anovel hyperbolic Radon inverse transformation, as shown in FIG. 6, onlythe desired primary reflections Re_(n) and the water-bottom reflectionWR are retained.

Example 2 West Texas Production Area

A seismic survey was done in the West Texas area of the United States.FIG. 7 shows a portion of the data collected along a two-dimensionalsurvey line. It is displayed in the offset-time domain and has beengathered into a common midpoint geometry. It will be noted that the datainclude a significant amount of unwanted noise, including mutedrefractions appearing generally in an area 30, aliased ground rolloccurring in area 31, and apparent air wave effects at areas 32.

In accordance with the subject invention, a conventional semblanceanalysis was performed on the data. The semblance plot was then used toidentify a stacking velocity function and, based thereon, to define afilter that would preserve primary reflection events and eliminatenoise.

The data of FIG. 7 were also transformed into the time-slowness domainwith a high resolution, hyperbolic Radon transformation of the subjectinvention using an offset weighting factor x^(0.5). A time variant,high-low filter defined through the semblance analysis was applied tothe transformed data, and the data were transformed back into theoffset-time domain using a hyperbolic Radon inverse transform and aninverse of the offset weighting factor p^(0.5). The data then weredisplayed, which display is shown in FIG. 8.

In FIG. 8, seismic reflection signals appear as hyperbolas, such asthose indicated at 33 and 34 (troughs). It will be noted that theunwanted noise that was present in the unprocessed data of FIG. 7 asmuted refractions 30, aliased ground roll 31, and air wave effects 32 isno longer present in the processed display of FIG. 8.

The data of FIG. 8 were then stacked along with other processed andfiltered CMP gathers from the survey in accordance with industrypractice. A plot of the stacked data is shown in FIG. 9. Seismicreflection signals appear as horizontal lines, e.g., those linesappearing at approximately 0.45 sec and 1.12 sec, from which the depthof geological formations may be inferred.

By way of comparison, the data of FIG. 7, which were not subject tofiltering by the subject invention, and from other CMP gathers from thesurvey were conventionally processed and stacked in accordance withindustry practice. The unfiltered, stacked data are shown in FIG. 10. Itwill be appreciated therefore that the stacked filter data of FIG. 9show reflection events with much greater clarity due to the attenuationof noise by the novel methods. For example, the reflection signalsappearing at 0.45 and 1.12 sec in FIG. 9 are not clearly present in FIG.10.

Example 3 West African Production Area

A marine seismic survey was done in a West African offshore productionarea. FIG. 11 shows a portion of the data collected along atwo-dimensional survey line. It is displayed in the offset-time domainand has been gathered into a common midpoint geometry. It will be notedthat the data include a significant amount of unwanted noise.

In accordance with the subject invention, a conventional semblanceanalysis was performed on the data. A plot of the semblance analysis isshown in FIG. 12. This semblance plot was then used to identify astacking velocity function and, based thereon, to define a filter thatwould preserve primary reflection events and eliminate noise, inparticular a strong water bottom multiple present at approximately 0.44sec in FIG. 12. The semblance analysis was also used to define lower andupper slowness limits p_(min) and p_(max) for transformation of thedata.

The data of FIG. 11 were also transformed, in accordance with theslowness limits p_(min) and p_(max), into the time-slowness domain witha high resolution, hyperbolic Radon transformation of the subjectinvention using an offset weighting factor x^(0.5). A time variant,high-low filter defined through the semblance analysis was applied tothe transformed data, and the data were transformed back into theoffset-time domain using a hyperbolic Radon inverse transform and aninverse of the offset weighting factor p^(0.5). The data then weredisplayed, which display is shown in FIG. 13.

In FIG. 13, seismic reflection signals appear as hyperbolas, such asthose starting from the near offset (right side) at approximately 1.7sec and 2.9 sec. It will be noted that the unwanted noise that waspresent in the unprocessed data of FIG. 11 was severely masking thereflection hyperbolas and is no longer present in the processed displayof FIG. 13.

The processed data of FIG. 13 then were subject to a conventionalsemblance analysis. A plot of the semblance analysis is shown in FIG.14. This semblance plot was then used to identify a stacking velocityfunction. By comparison to FIG. 12, it will be noted that the semblanceplot of FIG. 14 shows the removal of various noise events, including thestrong water bottom multiple present at approximately 0.44 sec in FIG.12. Thus, it will be appreciated that the novel processes allow for amore precise definition of the stacking velocity function which then canbe used in subsequent processing of the data, for example, as describedbelow.

The data of FIG. 13, along with processed and filtered data from otherCMP gathers in the survey, were then stacked in accordance with industrypractice except that the stacking velocity function determined throughthe semblance plot of FIG. 14 was used. A plot of the stacked data isshown in FIG. 15. Seismic reflection signals appear as generallyhorizontal lines, from which the depth of geological formations may beinferred. For example, the reflective signals starting on the right sideof the plot at approximately 0.41, 0.46, 1.29, and 1.34 sec. It will benoted, however, that the reflection signals in FIG. 15 slope and haveslight upward curves from left to right. This indicates that thereflective formations are somewhat inclined, or what it referred to inthe industry as “dip”.

By way of comparison, the unprocessed data of FIG. 11, along with datafrom other CMP gathers, were conventionally processed and then stackedin accordance with industry practice. The unfiltered, stacked data areshown in FIG. 16. It will be appreciated therefore that the stackedfilter data of FIG. 15 show reflection events with much greater claritydue to the attenuation of noise by the novel methods. In particular, itwill be noted that the reflection signals at 0.41 and 0.46 sec in FIG.15 are not discernable in FIG. 16, and that the reflection signals at1.29 and 1.34 sec are severely masked.

Example 4 Viosca Knoll Production Area

A marine seismic survey was done in the Viosca Knoll offshore region ofthe United States. FIG. 17 shows a portion of the data collected along atwo-dimensional survey line. It is displayed in the offset-time domainand has been gathered into a common midpoint geometry. It will be notedthat the data include a significant amount of unwanted noise, includingsevere multiples created by short cable acquisition.

In accordance with the subject invention, a conventional semblanceanalysis was applied to the data. A plot of the semblance analysis isshown in FIG. 18. This semblance plot was then used to identify astacking velocity function and, based thereon, to define a filter thatwould preserve primary reflection events and eliminate noise, inparticular the multiples caused by short cable acquisition that arepresent generally from approximately 2.5 to 7.0 sec. The stackingvelocity function is represented graphically in FIG. 18 as line 40. Thesemblance analysis was also used to define lower and upper slownesslimits p_(min) and p_(max) for transformation of the data.

The data of FIG. 17 were also transformed, in accordance with theslowness limits p_(min) and p_(max), into the time-slowness domain witha high resolution, hyperbolic Radon transformation of the subjectinvention using an offset weighting factor x^(0.5). A time variant,high-low filter defined through the semblance analysis was applied tothe transformed data, and the data were transformed back into theoffset-time domain using a hyperbolic Radon inverse transform and aninverse of the offset weighting factor p^(0.5). The data then weredisplayed, which display is shown in FIG. 19.

In FIG. 19, seismic reflection signals appear as hyperbolas, such asthose beginning at the near offset (right side) below approximately 0.6sec. It will be noted that the unwanted noise that was present in theunprocessed data of FIG. 17 below 0.6 sec and severely masking thereflection hyperbolas is no longer present in the processed display ofFIG. 19. In particular, it will be noted that the reflection signalsbeginning at the near offset at approximately 2.0 and 2.4 sec in FIG. 19are not discernable in the unfiltered data of FIG. 17.

The processed data of FIG. 19 then were subject to a conventionalsemblance analysis. A plot of the semblance analysis is shown in FIG.20. By comparison to FIG. 18, it will be noted that the semblance plotof FIG. 20 shows the removal of various noise events, including thestrong multiples located generally from 2.5 to 7.0 sec in FIG. 18 thatwere created by short cable acquisition. The semblance plot was thenused to identify a stacking velocity function, which is representedgraphically in FIG. 20 as line 41. Thus, it will be appreciated that thenovel processes allow for a more precise definition of the stackingvelocity function which then can be used in subsequent processing of thedata, for example, as described below.

The data of FIG. 19, along with the data from other CMP gathers that hadbeen processed and filtered with the novel methods, were then stacked inaccordance with industry practice except that the stacking velocityfunction determined through the semblance plot of FIG. 20 was used. Aplot of the stacked data is shown in FIG. 21. Seismic reflection signalsappear as horizontal lines, from which the depth of geologicalformations may be inferred. In particular, the presence of various trapspotentially holding reserves of oil or gas appear generally in areas 42,43, 44, 45, and 46, as seen in FIG. 21A, which is an enlargement of area21A of FIG. 21.

By way of comparison, the data of FIG. 17 and data from other CMPgathers in the survey were processed in accordance with conventionalmethods based on parabolic Radon transformations. In contrast to thesubject invention, the data were first NMO corrected. The NMO correcteddata then were transformed, without any transformation limits, to thetau-P domain using a low resolution parabolic Radon transformation thatdid not apply an offset weighting factor x^(n). A conventional mutefilter then was applied, and the preserved data were transformed backinto the offset-time domain using a conventional parabolic Radon inversetransform that did not apply an inverse offset weighting factor p^(n).The NMO correction was reversed to restore the preserved data, and theywere subtracted from the original data in the respective gathers.

The data that have been processed and filtered with the parabolic Radonforward and inverse transforms were then stacked in accordance withindustry practice. A plot of the stacked data is shown in FIG. 22.Seismic reflection signals appear as horizontal lines, from which thedepth of geological formations may be inferred. It will be noted,however, that as compared to the stacked plot of FIG. 21, reflectionevents are shown with much less clarity. As may best be seen in FIG.22A, which is an enlargement of area 22A in FIG. 22, and whichcorresponds to the same portion of the survey profile as shown in FIG.21A, traps 42, 43, 44, 45, and 46 are not revealed as clearly by theconventional process.

The foregoing examples demonstrate the improved attenuation of noise bythe novel methods and thus, that the novel methods ultimately allow formore accurate inferences about the depth and composition of geologicalformations.

While this invention has been disclosed and discussed primarily in termsof specific embodiments thereof, it is not intended to be limitedthereto. Other modifications and embodiments will be apparent to theworker in the art.

1. A method of processing seismic data to remove unwanted noise frommeaningful reflection signals indicative of subsurface formations,wherein said seismic data comprise amplitude data recorded over time ata number of seismic receivers in an area of interest, said amplitudedata are assembled into common geometry gathers in an offset-timedomain, and said amplitude data comprise primary reflection signals andcoherent noise that overlap in an offset-time domain; said methodcomprising the step of enhancing the separation between primaryreflection signals and coherent noise by transforming said data from theoffset-time domain to the time-slowness domain, wherein: (a) an offsetweighting factor x^(n) is applied to said amplitude data, wherein x isoffset and 0<n<1; and (b) said offset weighted amplitude data aretransformed from the offset-time domain to the time-slowness domainusing a Radon transformation; (c) whereby said primary reflectionsignals and said coherent noise transform into different regions of thetime-slowness domain to enhance the separation therebetween.
 2. Themethod of claim 1, wherein said assembled data are uncorrected fornormal moveout.
 3. The method of claim 1, wherein n in said offsetweighting factor x^(n) is approximately 0.5.
 4. The method of claim 3,wherein said assembled data are uncorrected for normal moveout.
 5. Themethod of claim 1, wherein said assembled amplitude data are transformedusing a hyperbolic Radon transformations.
 6. The method of claim 5,wherein said assembled data are uncorrected for normal moveout.
 7. Themethod of claim 1, wherein said offset weighting factor x^(n) is appliedand said amplitude data are transformed with a continuous Radontransformation defined as follows:u(p, τ) = ∫_(−∞)^(∞)𝕕x ∫_(−∞)^(∞)𝕕t d(x, t) x^(n) δ[f(t, x, τ, p)] or adiscrete Radon transformation approximating said continuous Radontransformation, where u(p,τ)=transform coefficient at slowness p andzero-offset time τ d(x,t)=measured seismogram at offset x and two-waytime t x^(n)=offset weighting factor (0<n<1) δ=Dirac delta functionƒ(t,x,τ,p)=forward transform function.
 8. The method of claim 7, whereinsaid assembled data are uncorrected for normal moveout.
 9. The method ofclaim 7, wherein said forward transform function, ƒ(t,x,τ,p), isselected from the transform functions for linear slant stack, parabolic,and hyperbolic kinematic travel time trajectories, which functions aredefined as follows: (a) transform function for linear slant stack:ƒ(t,x,τ,p)=t−τ−px (b) transform function for parabolic:ƒ(t,x,τ,p)=t−τ−px ² (c) transform function for hyperbolic:ƒ(t,x,τ,p)=t−√{square root over (τ² +p ² x ²)}.
 10. The method of claim9, wherein said assembled data are uncorrected for normal moveout. 11.The method of claim 1, wherein said offset weighting factor x^(n) isapplied and said amplitude data are transformed with a continuoushyperbolic Radon transformation defined as follows:${u\left( {p,\tau} \right)} = {\int_{- \infty}^{\infty}{{\mathbb{d}x}\; x^{n}\;{d\left( {x,\sqrt{\tau^{2} + {p^{2}x^{2}}}} \right)}}}$or a discrete hyperbolic Radon transformation approximating saidcontinuous hyperbolic Radon transformation, where u(p,τ)=transformcoefficient at slowness p and zero-offset time τ x^(n)=offset weightingfactor (0<n<1).
 12. The method of claim 11, wherein said assembled dataare uncorrected for normal moveout.
 13. The method of claim 1, whereinsaid offset weighting factor x^(n) is applied and said amplitude dataare transformed with a discrete Radon transformation defined as follows:${u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}{\sum\limits_{k = 1}^{N}{{d\left( {x_{l},t_{k}} \right)}\; x_{l}^{n}\;{\delta\left\lbrack {f\left( {t_{k},x_{l},\tau,p} \right)} \right\rbrack}\;\Delta\; x_{l}\;\Delta\; t_{k}}}}$where u(p,τ)=transform coefficient at slowness p and zero-offset time τd(x_(l),t_(k))=measured seismogram at offset x_(l) and two-way timet_(k) x_(l) ^(n)=offset weighting factor at offset x_(l)(0<n<1) δ=Diracdelta function ƒ(t_(k),x_(l),τ,p)=forward transform function at t_(k)and x_(l)${{\Delta\; x_{l}} = {{\frac{x_{l + 1} - x_{l - 1}}{2}\mspace{14mu}{for}\mspace{14mu} l} = 0}},{\pm 1},\ldots\mspace{11mu},{\pm \left( {L - 1} \right)}$Δx_(L)=x_(L)−x_(L−1) Δx_(−L)=x_(−L+1)−x_(−L)${{\Delta\; t_{k}} = {{\frac{t_{k + 1} - t_{k - 1}}{2}\mspace{14mu}{for}\mspace{14mu} k} = 2}},\ldots\mspace{11mu},{N - 1}$Δt₁=t₂−t₁ Δt_(N)=t_(N)−t_(N−1).
 14. The method of claim 13, wherein saidassembled data are uncorrected for normal moveout.
 15. The method ofclaim 13, wherein said forward transform function, ƒ(t _(k),x_(l),τ,p),is selected from the transform functions for linear slant stack,parabolic, and hyperbolic kinematic travel time trajectories, whichfunctions are defined as follows: (a) transform function for linearslant stack:ƒ(t _(k),x_(l) ,τ,p)=t _(k) −τ−px _(l) (b) transform function forparabolic:ƒ(t _(k),x_(l) ,τ,p)=t _(k) −τ−px _(l) ² (c) transform function forhyperbolic:ƒ(t _(k),x_(l) ,τ,p)=t _(k)−√{square root over (τ² +p ² x _(l) ²)}. 16.The method of claim 15, wherein said assembled data are uncorrected fornormal moveout.
 17. The method of claim 1, wherein said offset weightingfactor x^(n) is applied and said amplitude data are transformed with adiscrete hyperbolic Radon transformation defined as follows:${u\left( {p,\tau} \right)} = {\sum\limits_{l = {- L}}^{L}{x_{l}^{n}\;{d\left( {x_{l},\sqrt{\tau^{2} + {p^{2}x_{l}^{2}}}} \right)}\;\Delta\; x_{l}}}$where u(p,τ)=transform coefficient at slowness p and zero-offset time τx_(l) ^(n)=offset weighting factor at offset x_(l)(0<n<1)${{\Delta\; x_{l}} = {{\frac{x_{l + 1} - x_{l - 1}}{2}\mspace{14mu}{for}\mspace{14mu} l} = 0}},{\pm 1},\ldots\mspace{11mu},{\pm \left( {L - 1} \right)}$Δx_(L)=x_(L)−x_(L−1) Δx_(−L)=x_(−L+1)−x_(−L).
 18. The method of claim17, wherein said assembled data are uncorrected for normal moveout.